main: Skip command autocomplete if space-prefixed

Fixes #300

host.go

@@ -231,7 +231,7 @@ func (h *Host) AutoCompleteFunction(u *message.User) func(line string, pos int,
 		posPartial := pos - len(partial)
 
 		var completed string
-		if isFirst && strings.HasPrefix(partial, "/") {
+		if isFirst && strings.HasPrefix(line, "/") {
 			// Command
 			completed = h.completeCommand(partial)
 			if completed == "/reply" {